The exemplary embodiments relate generally to rotor assemblies for gas turbine engines and more particularly to methods and systems for assembling rotor assemblies.
A gas turbine engine is an example of a large rotary machine requiring dimensional precision for reducing vibration at high rotational speed. Vibration may occur due to mass unbalance around an axial centerline axis of the engine, and/or due to eccentricity of the rotor therearound. Runout, roundness, concentricity and flatness are of particular concern in an assembly of rotor components since they may contribute to eccentricity. The individual rotors in a typical gas turbine engine vary in configuration for aerodynamic, mechanical, and aeromechanical reasons, which increases the complexity of the engine design and the difficulty in reducing undesirable eccentricity.
For example, a multistage compressor or turbine includes rows of airfoils extending radially outwardly from supporting rotor disks. The airfoils may be removably mounted in corresponding dovetail slots formed in the perimeter of the disks, or may be integrally formed therewith in a unitary construction known as a blisk. Individual disks may be bolted together at corresponding annular flanges having a row of axial bolt holes through which fastening bolts extend for joining together the several rotors in axial end-to-end alignment. Some rotor disks are typically formed in groups in a common or unitary rotor drum, with the drum having end flanges bolted to adjoining rotors having similar annular flanges. Accordingly, the multistage assembled rotor includes several rotor disks axially joined together at corresponding annular flanges. Each rotor is separately manufactured and is subject to eccentricity between its forward and aft mounting flanges, and is also subject to non-perpendicularity or tilt of its flanges relative to the axial centerline axis of the engine.
Both eccentricity and tilt of the rotor end flanges are random and typically limited to relatively small values. However, the assembly of the individual rotors with their corresponding flange eccentricities and tilts are subject to stack-up and the possibility of significantly larger maximum eccentricity due to the contribution of the individual eccentricities. Accordingly, when the rotor assembly is mounted in bearings in the supporting engine stator, the corresponding rotor seats or journals mounted in the bearings may have relative eccentricity therebetween, and intermediate flange joints between individual rotors of the assembly may have an eccentricity from the engine centerline axis which exceeds the specified limit on eccentricity for the rotors due to stack-up. In this case, the rotor assembly must be torn down and reassembled in an attempt to reduce stack-up eccentricities to an acceptable level within specification.
One manner of reducing the random nature of the assembly stack-up is to measure each rotor during the assembly sequence to determine the runout, roundness, concentricity and flatness of mating diameters and flanges and then assembling that component to a preceding component for reducing the collective stack-up of eccentricity upon final rotor assembly. Individual rotors are mounted on a turntable using a suitable fixture so that the rotor may be rotated about its axial centerline axis. Linear measurement gauges are mounted to the table and engage the corresponding mounting flanges of the rotor for measuring any variation of radius of the flanges from the axial centerline axis around the circumference of the flanges, and for measuring any variation in axial position of each of the flanges around the circumference.
The gauges are operatively joined to a computer, which receives the measurement data from the gauges mounted at each end flange during measurement. The computer is programmed to calculate various geometric parameters for the end flanges. In particular, the radial measurement data may be used to determine high and low points on the flanges. The computer may then determine a mating rotor surface based on the high and low points of the measured rotors. The computer may also utilize a least squares center algorithm to determine a best-fit surface. This algorithm provides a single vector representing the slope of a face surface or eccentricity of a flange surface. The computer may then determine a best-fit based on vectors from multiple rotors and assemble them accordingly. These methods may ignore other opportunities for an optimal assembly. For example, accuracy of processing of least squares center and high/low points may be improved to properly account for all shapes/conditions experienced. The least squares center results are a simplified description of the average surface. It presents a best-fit model not taking into account local variations in the topography of the mating surfaces and diameters, which may have a significant impact on the stack. For example, if a rotor with a flange face with two equal and substantial peaks 180 degrees apart was mated to a perfectly flat part, it could be rocked to one side or the other, pivoting about the peaks, depending on which side was attached first. Using the same example with two peaks, if the mating part had a similar feature (two peaks), the computer does not optimize the stack by looking at an interlocking of peaks.